1. Field of the Invention
The invention broadly relates to optical servo systems. More particularly, the invention relates to the optical detection system used in an optically assisted disk drive to detect marks on a magnetic disk and thereby precisely locate the magnetic read/write head relative to tracks on the magneto-optical disk.
2. Brief Description of the Prior Art
Since the introduction of the personal computer in the 1970s and the development of the floppy disk, the need for greater and greater amounts of storage space has continued unabated. The original floppy disk could store less than 100 kilobytes and the most commonly used (3.5 inch) floppy disk today, introduced in the late 1980s can store 1.4 megabytes. Although fixed (hard) disks now store many gigabytes, there remains a need for removable storage media with high capacity.
High capacity removable storage media became popular in the 1980s with the advent of desktop publishing (DTP). Relatively large, clumsy, and undependable “cartridges” from Syquest, Iomega, and other companies were used to transport large DTP files that could not fit on a floppy disk, to a printing plant. High capacity storage media is still in demand today for transporting large files when a broadband connection is not available and for transporting confidential information without using the public network.
One high capacity removable media system which is growing in popularity is the “a:drive” from OR Technology Inc. of Campbell, Calif. While its outward appearance is almost indistinguishable from that of a 3.5 inch, 1.44 megabyte floppy disk drive, the “a:drive” provides 120 megabytes of storage on ultra high density disks, known as “LS-120” or “Superdisk” media. At the same time, the “a:drive” product is compatible with current and legacy 3.5 inch technology and can read and write to both 720 kilobyte and 1.44 megabyte disks. As its name implies, the “a:drive” can serve as a bootable drive in any system in which it is installed.
The “a:drive” achieves its high capacity and enhanced accuracy and reliability by using an optical positioning system for accurately guiding a magnetic dual-gap head that accommodates the differing track densities of conventional and ultra high density disks without error or mishap.
Prior art FIG. 1 shows a dual media disk storage system for reading data from and writing data to the surface 10 of removable magnetic media 12 having an axis of rotation 14 and a plurality of concentric data tracks 16. Although the disk drive system is capable of handling dual media, in this instance, for the sake of clarity, only one disk is shown, the well known 3.5 high density type that holds 1.44 megabytes when formatted.
A read/write head 18 is guided by an actuator 20 and actuator arm 22 which positions the read/write head 18 over a desired track 16 on the surface 10 of disk 12. The actuator arm 22 carries a strip having a periodic reflection profile 24 which is used in this instance because the 3.5 disks do not carry any location markings on their surface. In this instance, the periodic reflection profile 24 is a linear encoder. Actuator 20 is under control of a conventional, closed loop servo system 28 which is responsive to a signal from an optical sensor 30 mounted on the underside of sensor housing 28.
FIG. 2 shows in more detail how a split beam arrangement is used to detect either the reflection profile for a linear encoder when reading/writing 3.5 disks or the markings on the surface of an LS-120 disk when reading/writing it. The sensor system carried on the arm 22 includes, in addition to the light detector 30, a laser source 32, a hologram 34, a lens array 36 and a rooftop mirror 38. Light from the laser source 32 is diffracted by the hologram 34 and focused by the lens array 36. The rooftop mirror directs the light and reflections to either the linear encoder 24 or the surface of an LS-120 disk 40.
It can be appreciated from prior art FIG. 2 that the sensor system requires multiple passive optical elements, all of which must be aligned during the assembly process. The alignment requires expensive tooling. Each passive element occupies a finite space and additional space must be provided for the alignment tooling. The sizes of the elements also require a large mechanical supporting structure.
In addition, it will be appreciated by those skilled in servo system shown in prior art FIG. 2 is relatively large and with a “full height” drive bay to be accommodated.
Additionally those skilled in the art will recognize the servo system for an LS-120 type disk drive requires a quadrature between the adjacent sensors; that the detection of the sensors m synchronous; and that the first stage of the pre-amplifier used in a typical optical servo system would be more effective by amplifying the tangential and radial tracking signals only and not the DC component, thereby allowing for a larger gain and more signal amplitude.
Co-owned application Ser. No. 09/591,930, now U.S. Pat. No. 6,836,451, discloses one solution to the problems of the prior art devices. In both the prior art and the co-owned co-pending application, light from a laser source is focused by passive optical elements to form three highly focused points of light at the plane of the disk. Other passive optical elements gather reflected light from these three points and focus the reflected light onto three separate detectors. The detectors produce track sensing signals that include a low noise pair of sinusoidal signals in exact quadrature indicating the track radial position.